Circular  no.  26.  Second  Seri(S. 

I  iiiicil  States  DiMuiiiiitMn 


DIVISION   OF    KN  IOMOI.OGY. 


I'll i:  PEAH  SL1  ».. 

/  rnul,  s     It  11,11,  i  mi     |;> 

i  M  \i;  \i   ii  BIS  in  -    \\i>   in-  ["ORY 

The  damage  to  the  foliage  i>f  the  pear,  cherry,  plum,  an 
from  the  slimy  slug-worm  is   familiar  t<>  every  fruit  grower.     Tun  or 
three  generations  of  these   Blug-worms,  or  ' ' -In  ■  also 

termed,  appear  during  the  Bummer  and  frequently  in  Buch  extraordi- 
nary numbers,  with  the  later  broods,  thai  the  leaves  "i  the  attacked 
plants  turn  brown,  die,  and  tall  to  the  ground  in  midsummer,  and  tin- 
new  growth  "I  foliage  which 
ifterward  thrown  out  is 
o ft  e n  similarly  destroyed. 
Trees  thus  denuded  air  much 
checked  in  growth  <>r  greatly 
injured,  if  not  killed.  When 
the  -lu'js  are  very  abundant, 
as  they  frequently  are  in  July 
during  the  second  brood,  the 
Bound  of  the  eating  of  myri- 
ads of  mouths  resembles 
somewhat  the  falling  of  line 
misl  <>r  rain  on  the  leave-, 
and  instead  of  one  or  two 
larva  at  work  on  a  leaf  there 
may  he  upward  of  thirty. 
Under  such  circumstam 
very  distincl  and  disagi 
able  odor  is  disseminated  by  the  multitudes  of  slimy  slug-like  creatures. 

The  slug-fly  is  a  -mall,  glossy  black  insect,  considerably  less  in 
than  the  house  fly,  measuring  only  aboul  one-fifth  of  an  inch  in  length. 
The  win:;-,  which  are  four  in  number,  are  transparent,  iridescent,  and 
have  a  -moky  Kin. I  across  the  middle,  which  varies  in  intensity  in  dif- 
ferent specimens.  It  belongs  to  the  family  commonly  termed  "  saw- 
flies,"  (Tenthredinida  )  on  account  of  the  Baw-like  instrument  or 
ovipositor  with  which  the  female  insect  places  it-  eu'i:-  in  the  leavi 
other  -oft  parts  of  the  plant. 


>  adult    -;r.v 

» iih  sltmi  In   norma 

1 


2 

The  pear  blag  is  an  old  enemy  of  fruit  trees,  and  while  it  has  been 
known  in  this  country  for  over  a  hundred  years  as  an  American  species, 
the  interesting  fact  that  it  is  the  familiar  slug-worm  pest  of  the  pear  and 
plum  trees  of  Europe  has  not  hitherto  been  definitely  ascertained.  In 
Massachusetts,  toward  the  latter  pari  of  the  last  century,  this  insect 
became  very  destructive,  and  Prof.  Win.  I).  Peck  prepared  an  admira- 
ble, illustrated  account  of  it  under  the  title  of  "  Natural  History  of  the 
Slug  Worm,"  which  was  printed  in  Boston  in  17!'!)  by  the  Massachu- 
setts Society  for  the  Promotion  of  Agriculture.  The  author  obtained 
for  the  production  the  society's  premium  of  $50  and  a  gold  medal.  The 
account  of  this  insect  given  by  Professor  Peck  is  fairly  complete  and 
accurate,  hut  for  fifty  years  it  has  been  out  of  print  and  inaccessible, 
and  I  have  not  been  able  to  consult  it.  Harris  abridged  and  condensed 
it  for  his  account  of  the  slug-worm  in  his  "Insects  Injurious  to  Vegeta- 
tion." published  in  1841,  and  later  writers  have  generally  followed 
Harris. 

In  Europe  this  insect  was  one  of  the  first  of  the  injurious  species  that 
attracted  attention,  and  one  of  the  earliest  of  the  economic  writers  on 
insects,  Reaumur,  in  the  fifth  volume  of  his  bulky  work  published  in 
ITllt,  gives  a  short  account  of  it,  together  with  recognizable  figures  of 
the  insect  in  different  stages,  illustrating  also  the  effect  of  its  work  on 
leaves.  The  first  description  and  name  which  stands  out  unquestioned 
is  by  Retzius  (1783),  who  describes  the  species  as  Tenthredo  limacina. 

Linn'e,  in  descrihing  what  seems  to  have  been  an  entirely  distinct  in- 
sect, which  he  called  Tenthredo  cerasi,  quotes  Reaumur's  account  of  the 
slug-worm,  which  he  erroneously  took  as  belonging  to  his  species,  and 
very  generally  since,  therefore,  the  former  has  been  called  cerasi  Linn. 
(Cameron.) 

Professor  Peck  in  descrihing  the  insect  gave  it  the  name  suggested  by 
Linne,  and  was  of  the  belief  that  the  species  occurring  about  Boston 
was  at  most  a  mere  variety  of  Linne's  species,  meaning,  however,  the 
common  pear-slug  fly  of  Europe.  Later  American  writers  have  treated 
it  as  a  native  species,  and  under  the  generic  name  of  Selandria  or  Erio- 
campa,  Peck  has  had  the  credit  lor  it  in  this  country,  and  the  fact 
that  Peck  himself  associated  it  with  Linne's  species  has  been  generally 
overlooked. 

That  the  species  so  common  in  this  country  is  identical  with  the  slug- 
fly  of  Europe  has  been  fully  established  by  a  comparison  of  specimens 
from  Europe  with  abundant  American  material. 

In  Europe  this  insect  has  heen  the  suhject  of  description  by  innu- 
merable authors,  having  received  at  least  nine  different  specific  names 
and  having  heen  referred  to  some  eight  genera.  In  this  country  it  has 
also  heen  the  suhject  of  many  short  notices,  hut  of  very  few  full  accounts 
other  than  the  one  published  by  Peck. 


The  i"  ii     ! !"  pel  which   i     en  ilj  di  tributed  \\  itli   the  Boil 

about  the  pi  ante  which  il    infests,  and   it    hue  I  ried  about  the, 

world  until  it  has  made  its  waj  into  practii  allj  everj  civilized  country. 
Ii  is  known  at  least  td  occur  throughout  Europe  and  America  and  in 
many  of  the  British  colonies.  The  Blugfi  have  been  round  on  11 
many  different  plants,  Reaumur  recording  them  on  the  plum  and  cherry, 
and  especially  pear,  bul  also  on  the  oak.  and  a  recent  catalogue  by 
Dal  la  Torre  indicates  their  occurrence  "n  over  thirtj  different  plants  in 
Europe  Certain  Btone  Fruits  and  the  pear,  particularly  the  lattei 
their  especial  fa>  nril  i 


I 


I 


-  -:""~--'- 


I 


I. II  I.    HISTORY      Wl>    II  M:i  i  8. 

TIic  parent  il\  may  !"•  seen  on   the  pear  or  othei   trees  which  Berve 
.1-  food  for  the  larva'  very  early  in  the  sprin   .     In  Washington,  D.  C, 
the  present   Beason  thej  were 
observed     on     the     new  \\  ex  flHPH 

panded  leaves  by  the  middle 
i>f   April   actively  engaged   in  / 

laj  ing  their  eggs.     In  the  lat-  /• 

itude  nt  Boston  and  northward 
the  flies  do  not  appear  much 
before  the  middle  of  May.  and 
the  egg-laying  is  chiefly  during 
the  lattei  pari  of  May  and  first 
nt'  June.  Judging  from  our 
experience  here  many  of  the 
laid  by  the  flies  which 
appear  in  April  perish  during 
the  cold  wet  weather  which 
often  characterizes  this  month. 
This  wa>  notably  the  case  the 
present  Beason,  few,  if  any,  of  the  eggs  first  deposited  producing  larva  . 

But  one  egg  is  deposited  in  a  place,  and  it  i-  always  inserted  From 
the  nniler  Bide  of  the  leaf.  The  ovipositor  is  thrust  obliquely  through 
the  leaf  to  the  upper  epidermis,  bul  not  piercing  the  latter,  and  Bhows 
there  distinctly  through  the  transparent  upper  skin  of  the  leaf,  as  rep- 
resented in  the  accompanying  illustration  (fig.  2),  while  the  insect 
titherw  ise  is  completely  hidden.  The  saw  -like  instrument,  when  brought 
into  the  position  noted,  is  moved  rapidly  with  a  swinging  lateral  motion 
from  siile  tn  Bide,  cutting  the  upper  epidermis  free  bo  a-  to  form  an 
irregular  cell  or  pockel  of  peculiar  flattened  ovoid  outline.  The  egg  is 
quickly  passed  down  between  the  plates  of  the  ovipositor  and  dropped 
into  the  pockel  thus  made,  the  time  occupied  being  a  little  ovei  one 
minute  for  the  entire  operation. 


I  lllustrattm 

and  emergence  ol  larva    a,  cutting  ol  cell  i 
epidermis,  w  Ith  ovipos  u  im- 

larged  (original). 


Usually  before  placing  the  egg  the  fly  runs  about  rapidly  over  the 
upper  surface  of  the  leaf,  examining  it  carefully,  and  then  goes  over  to 
the  under  side  to  insert  an  egg,  after  which  she  reappears  on  the  upper 
surface  of  the  leaf  and  rests  for  a  minute  or  so  before  flying  to  another 

leaf. 

it  often  happens  that  a  good  many  eggs  are  deposited  in  a  single 
leaf,  hut  I  am  convinced  that  this  is  usually  by  different  flies  or  at 
different  visits  by  the  same  individual.  The  loosened  epidermis  about 
the  egg  dries  somewhat,  and  the  egg-cell  soon  appears  as  a  minute 
brownish  spot  with  the  almost  colorless  egg  showing  at  the  center. 
'1'hc  egg  is  oval,  slightly  flattened  on  one  side,  and  remains  in  its 
peculiar  cell  (see  fig,  2.  I>)  for  a  period  of  about  two  weeks  before  the 
larva  escapes.  It  is  so  placed  in  the  leaf  that  it  can  be  readily  watched 
and  its  gradual  increase  in  size  by  absorption  from 
the  leaf  and  the  development  of  the  young  larva 
can  he  easily  studied  with  a  hand  lens. 

The  larva  emerges  on  the  upper  surface  of  the 
^|#  leaf  through  a  very  regular  semicircular  cut 
y  which  it  makes  near  the  center  of  the  cell.  At 
first  it  is  (dear  or  free  from  slime  and  in  color 
nearly  white,  except  the  yellowish-brown  head; 
hut  almost  immediately  the  slimy  or  gluey  olive- 
colored  liquid  begins  to  exude  over  its  entire 
body,  giving  it  the  appearance  of  a  minute  slug, 

Pro.  3.— Pear    slug:    «.    last  ...  -i     .•  ,  •    ,     •  ,,  •.  i. 

moulted  larval  skin:  6.  larva  '"'   Sof1    snal1'  from    wmch  ]t    gets  lts  name-      Its 

after    easting    last    skin-  head   is  dark   brown,  appealing  black  under  the 

somewhat    enlarged    (orig-  ,.  ,  .,       ,      -.        ,       ,  ,  ,, 

taal)  slime,  and  the  body  also  becomes  almost  equally 

dark.     The  anterior  segments  are  much  swollen. 

covering  up  and  concealing  the  head  and  thoracic  legs.      In  common  with 

other  saw-fly  larv;e  it  has  a  great  number  of  false  legs  or  prolegS  on  the 

abdominal  segments;  in  this  species  there  are  fourteen  such  prolegs,  the 

terminal  pair  being  wanting,  and  the  tapering  tip  of  the  body  is  usually 
slightly  elevated. 

As  soon  as  the  larva  emerges  from  the  egg  it  begins  feeding  on  the 
upper  surface  of  the  leaf,  eating  out  small  holes  or  patches  about  the 
size  of  a  pin  head  or  smaller,  hut  never  eating  entirely  through  the  leaf. 
The  larva'  feed  almost  invariahly  on  the  upper  side  of  the  leaves  and 
the  minute  eaten  spots  which  they  make  at  the  start  rapidly  increase  in 
si/.e  until  much  of  it.  hut  not  the  entire  surface  of  the  leaf,  is  denuded, 
leaving  merely  a  network  of  veins,  or  a  leaf  skeleton,  held  together  by 
a  nearly  intact  lower  epidermis.  Leaves  thus  eaten  turn  brown,  die. 
and  fall  to  the  ground,  the  tree  being  frequently  defoliated,  except  for 
the  effort  it  usually  makes  to  put  out  a  new  growth. 

The  characteristic  features  of  the   larva  are   its  swollen   anterior 


imiit-  and  its  olive-colored  slimj  covering,  which  last  i  probablj  a  pro 
ti.  thui  furnished  \>\  nature  against  the  attacke  ol  parasiti<  insects.  It 
is  \<r\  Bluggish  in  movement)  but  has  an  enormous  app<  Lite.  It-  growth 
is  rapid,  full  use  being  attained  within  considerably  less  than  .1  month, 
usually  about  twenty-five  days,  the  time  varying  a  little  with  the  nature 
ol  tin'  weather.  It  does  nol  alter  much  in  appearance  during  growth, 
and  ultimately  reaches  a  length  of  nearly  half  an  inch.  It  Bheds  it^ 
akin  four  times  during  its  larval  life,  and  usually  I  akin  foi 

its  first  meal  after  each  moult.  When  full  grown  it  moults  a  fifth  time, 
Imt  on  this  occasion  leaves  its  caBt  skin  as  a  Blender  line  of  Blime 
attached  to  the  leaf.  The  dark  olive  green  -limy  appearance  which  has 
hitherto  characterized  the  larva  is  losl  with  tl  moult  (see  fig.  3, 

a,  which  .-how-  last  moulted  b  kin),  and  in  its  stead  it  appears  as  a  lighl 
orange-yellow  worm,  perfectly  clean  and  dry,  with  the  head  lighl  col- 
ored and  only  the  mi  mite  circular  ey<  9pots  bla<  k  (fig.  '■'•.  b).  A  green- 
ish area  ahows  along  the  posterior  two-thirtls  of  the  body,  which  is 
merely  the  remains  of  the  last  meal  appearing  through  the  transparent 
>kin.  The  larva  due-  nol  feed  after  this 
moult,  hut  crawls  down  the  plant  to  the 
ground,  which  it  burrows  into  actively,  dis- 
appearing beneath  the  Burface  in  a  very  short 
time,  it  penetrates  to  a  depth  of  from  half 
an  inch  to  two  or  three  inches,  usually  the 
lesser  distance,  and  at  the  extremity  of  the    1  oon:  b, 

burrow  presses  the  soil  away  from  itself  so 

11  11  -i  enlarged  (oriirii 

as  to  form  a  little  cell  or  chamber,  the  Bidi  - 

of  which  it  moisten-  with  saliva.     The  drying  and  hardening  ol   the 

walls  of  the  chamber  form  a  sort  of f  turn  texture  and  more  ,,i 

less  impervious  to  water  (see  fig.  I.  a). 

During  the  heated  season  of  July  and  August  the  transformation  from 
the  larval  to  the  pupal  Btage  and  from  the  latter  to  the  adult  insect  is 
quite  rapid,  the  pupal  Btage  being  assumed  in  from  -ix  to  eighl  days, 
and  the  adult  flies  transforming  and  digging  out   through  the  soil  some 

twelve  Or  fifteen   day-   alter  the   larva   entered    it. 

It  ,-eem-  Iroin  the  studies  by  Peck,  and  in  part  confirmed  by  my  own 
observations,  thai  all  of  the  larva  of  the  spring  brood  do  not  transform 
at  once,  but  some  few  of  them  remain  unchanged  as  contracted  dormant 
larva' over  w  inter  to  transform  the  next  spring.  It  is  true  also  of  the 
Becond  broods  of  larva'  thai  some  of  them  come  oul  the  same  season, 
while  others  remain  over  winter  and  do  nol  pupate  until  Bhoitly  1 
the  appearance  of  the  adults  in  April  and  May.  The  holding  over  to 
the  next  year  of  certain  larva  of  each  brood  is  doubtless  a  provision  ol 
nature  to  prevent  the  extermination  of  the  species  by  any  untoward  ai  1  i- 
dent,  such  a-  the  absence  ol  food,  unfavorable  climatic  condition 


6 

abundance  of  natural  enemies  which  might  prove  disastrous  to  the  spe- 
cies should  all  appear  at  once. 

In  the  latitude  of  Washington,  Q.  ('..  the  first  brood  of  larva'  practi- 
cally all  disappear  from  the  trees  by  the  end  of  June,  and  the  first  Hies 
of  the  second  brood  begin  to  appear  about  June  20  and  are  out  in 
greatest  number  about  the  first  of  July.  It  is  the  progeny  of  tins 
second  hrood  of  flies  that  is  particularly  disastrous  to  the  trees. 
although  the  spring  brood  of  larvae  is  often  sufficiently  abundant  to  do 
very  serious  injury. 

NATURAL    ENEMIES. 

The  slimy  repellent  covering  of  the  larva  does  not  altogether  prevent 
its  being  preyed  upon  by  parasitic  insects,  and  in  Europe  some  half 
dozen  parasites  have  been  reared  from  it.  In  this  country  Peck  men- 
tions a  minute  parasitic  fly,  determined  by  Westwood  as  a  species  of 
Encyrtus,  which  stints  the  egg  of  the  slug-fly  through  the  upper  epi- 
dermis of  the  leaf,  placing  in  each  egg  of  its  host  a  single  one  of  its 
own — much  more  minute.  The  little  parasitic  maggot  when  it  hatches 
finds  food  enough  within  the  egg  of  the  slug-fly  for  the  needs  of  its  full 
development,  changes  to  the  chrysalis  therein,  and  ultimately  emerges 
a  perfect  fly  like  its  parent.  So  abundant  is  this  parasite  at  times,  as 
reported  by  Peck,  that  the  second  litter  of  eggs  is  sometimes  nearly  all 
destroyed.  I  have  found  evidence  of  the  occurrence  at  Washington. 
I).  C,  of  this  or  some  related  parasite,  hut  failed  to  secure  the  adult 
insect. 

REMEDIES   AND   PREVENTIVES. 

In  the  effort  to  exterminate  this  insect  the  slimy  covering  exuded  by 
the  larvae  in  such  copiousness  was  formerhy  taken  advantage  of  and 
applications  of  various  powders  and  dusts  were  made  to  them,  such  as 
ashes,  lime,  or  road  dust,  with  the  object  of  having  it  adhere  to  their 
viscid  surface  and  kill  them.  Under  this  treatment,  however,  the  larva 
usually  merely  sheds  the  incumbered  skin  and  starts  in  life  afresh  with 
a  new  coat. 

The  best  means  of  destroying  the  slug-worm  is  to  spray  the  plants 
with  an  arsenical  wash  or  with  a  simple  soap  solution.  The  larva'  are 
delicate  and  easily  killed,  and  as  they  eat  almost  exclusively  on  the 
upper  surface  of  the  leaf  where  the  poison  can  he  most  easily  placed, 
they  get  the  greatest  amount  of  it  and  are  the  easiest  of  all  larva'  to  he 
thus  exterminated.  The  plants  may  he  sprayed  with  Parks  green  or 
other  arsenical  wash  at  the  rate  of  1  pound  of  the  poison,  mixed  with 
an  equal  amount  of  lime,  to  250  gallons  of  water. 

The  soap  wash  to  he  effective  must  he  applied  at  a  strength  of  one- 
half  pound  of  soap  t<>  a  gallon  of  water,  first  dissolving  the  soap,  prefer- 
ably whale  oil.  by  boiling  in  a  small  quantity  of  water. 


Where  one  has  bu(   fe*  plants  to  rnraj  and  do. 
,„  arsenical  or  the  soap  wash,  hellebore  maj  be  used  either  as  a  drj 
powder  or  wa  wel  Bpray.     The  powder  maj  be  applied  with  a  bellows 
,„•  dusted  lightly  over  the  plants  from  a  cloth  bag,  making  the  applica- 
tion preferablj  when  the  plants  are  wel  with  dew. 

So  sensitive  is  tl             worm  that  very  1  nil  oftei 

,,     m,|  „   is  much  less  apl   to  be  injuriou    in  wel  us.     Fo.   this 

,n  h   maj  often  be  possible  1 1  plants  ol   il  bj  subjecting  them 


t,.  ;i  forcible  water  Bpray. 


C.    L.    M  IBLAl  I 
First   Assistant    Entomoh 


Approved  : 

.1  \\ii>    \\  ILSON, 

§    /■«  tary  of  Agrit  ulturi  . 
\\  L8UINOTON,  D.  C,  August   :'s-    1897 


o 


UNIVERSITY  OF  FLORIDA 

III  I  M 

3  1262  09216  4838 


